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More Than Just Dry Air: Controlling Oxidation and Intermetallics in Moisture-Sensitive Devices

To avoid the damage of micro-cracks and delamination during the processing of electronic components, appropriate environmental storage is essential. The introduction of lead-free soldering and the associated higher processing temperatures involved makes moisture management even more important. Lead-free reflow increases the consequent saturated vapor pressure within components considerably (up to 30 bars). The same component that could be safely processed before lead-free becomes a moisture sensitive device with limited floor life. The difference is often two sensitivity levels higher classification (MSL) and shorter allowable exposure time (“floor life”).

Component suppliers should deliver these moisture sensitive components in effective protective packaging to avoid absorption of humidity during transport and storage. These moisture barrier bags (MBB) are made from multiple layers of plastic and aluminum. Properly prepared and sealed, they are also a protective packaging that can prevent oxidation. ESD bags or zippered plastic bags do not protect against moisture. After opening the package, the time begins during which the components absorb humidity. Depending upon ambient humidity and temperature, the components can be safely used only within a limited time period. This time period is classified by the IPC/JEDEC J-Std 033C.

When a component has exceeded the allowed exposure time the component can be dried and made safe again through a baking process, traditionally done at 125°C. The component should be processed especially carefully after that. A repeated absorption of humidity must be avoided because the baking process should not be repeated.
Even one exposure to baking at these temperatures induces oxidation and inter-metallic growth, which reduces the wetting ability of the connection surfaces. Intermetallic thickness has been shown to increase by approximately 50% when baking at 125°C for four days. Thicker inter-metallic layers can lead to a reduction in solder joint integrity and in extreme cases reduce solderability.

To fight this well-known effect, many suppliers of baking ovens provide an additional reduction of oxygen by means of a nitrogen atmosphere or vacuum during the drying process. Setting the clock back to zero for the component can take in excess of 72 hours, inevitably bringing along considerable costs for nitrogen, and only a low rest-oxygen content of less than 13 ppm stops the oxidation.

Lead-Free Soldering Alloys 
Because of the considerably higher content of tin in lead-free soldering alloys, the need to consider oxidation protection during storage has increased in importance. This is caused by higher oxidation tendencies of these alloys and the generally more difficult wet ability and flow properties of lead-free soldering alloys.

The Oxidation Process
The oxygen causing the oxidation originates from two different sources. The first is the oxygen molecule, found world-wide in our atmosphere. However, because of its strong atomic bond it only occurs at temperatures higher than 40°C. The second and in fact more aggressive bearer of oxygen is the water molecule. Here, the oxygen atom weakly connected, and considerable oxidation can be observed at low temperatures. This means that not the content of oxygen, far more the content of humidity is decisive for the oxidation percentage in stored components. Technically, it is possible to solve both problems at the same time. However, it is important to avoid heating above 40°C thereby eliminating the air-oxygen as a reaction partner, and to provide a strong dehumidification of the air at the same time. To achieve this, dry storage systems have been designed that can produce internal atmospheres of below 1% RH. With this extremely low content of humidity it is possible to protect the components against the additional absorption of moisture and also to remove the moisture already absorbed. As the diagram below shows, even storage in very clean nitrogen does not provide actual dehumidification of components as levels under 0.1 Wt % are not possible.

To read this entire article, which appeared in the January 2017 issue of SMT Magazine.